This disclosure relates generally to the field of scintillation type radiation detectors. More specifically, the invention relates to methods for adjusting spectral analysis of radiation detected with scintillation counters for the effects of “pile up” on the analyzed radiation spectra.
Certain types of well logging instruments have gamma ray radiation detectors capable of making measurements related to the numbers of and the energy of detected gamma rays. The gamma rays may be naturally occurring or may result from interaction of radiation emitted into formations surrounding a wellbore by a chemical radioisotope or an electrically controlled source, such as an x-ray generator of neutron generator of types well known in the art.
The energy of the detected gamma rays may be determined using a scintillation detector coupled to a photomultiplier. A pulse height analyzer or multichannel analyzer (MCA) coupled to the output of the photomultiplier. Present versions of the MCA include “integrator” type MCAs, wherein each analyzer channel integrates the amplitude of all pulses occurring within a selected voltage range and within a selected time window. When detected events are too closely spaced in time to be separately identifiable, the result is known as “pileup.” The response of modern integrator-type MCA to multiple detection events is much simpler than what was encountered with earlier ramp analog to digital convertors (ADC's), which just determined the maximum of each voltage pulse from the photomultiplier to assign each pulse to a selected energy level channel. An integrator-type MCA will produce a summed response nearly identical to the sum of the responses that it would have produced for each individual pulse had they been adequately separated in time; whereas, the response of a ramp ADC would depend on the exact time separation of the multiple events.
Measurement of gamma-ray energy spectra, regardless of how those gamma rays are produced, but especially to measurements made at high count rates, where multiple events detected closely in time may be included in the measured spectrum as single detection events with an apparent energy equal to or approaching the sum of the energies of the closely-spaced events benefit from removal of pile up distortion in the acquired spectra. Because of the random nature of the time coincidence of multiple detection events in pileup, pileup events contain virtually no useful information. Such events merely add erroneous detection events to the overall gamma ray detection spectrum leading to the desire to remove them from the spectrum. There is a need for methods to remove pileup events that had not been recognized as being too closely spaced in time and thus not rejected by the data acquisition system.